Memory array with low power bit line precharge

ABSTRACT

An integrated circuit memory array with an extra column of memory cells and a reference bit line is provided, in which the reference bit line acts as a reference for a shared precharging and clamping control on the bit lines in the array. Clamp transistors are coupled to respective bit lines in the array, and adapted to prevent voltage on the respective bit lines from exceeding a target level. A comparator has an input coupled to the reference bit line and an output coupled to the clamp transistors on the plurality of bit lines. The comparator generates a bias voltage which turns on the clamp transistors at a first bias level when the reference bit line has a voltage below the target level, and a second bias level, which is lower than the first bias level, when the reference bit line has a voltage near the target level.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to integrated circuit memory devices, andto sense circuitry in such memory devices.

2. Description of Related Art

Integrated circuit memory devices are becoming smaller and faster. Onelimitation on the size and speed of memory devices arises from circuitryused for precharging and biasing bit lines in preparation for sensingdata from the array. Typical structures used for these purposes areillustrated in U.S. Pat. No. 6,219,290, entitled MEMORY CELL SENSEAMPLIFIER, invented by Chang et al.; U.S. Pat. No. 6,498,751, entitledFAST SENSE AMPLIFIER FOR NONVOLATILE MEMORIES, invented by Ordonez, etal.; and U.S. Pat. No. 6,392,447, entitled SENSE AMPLIFIER WITH IMPROVEDSENSITIVITY, invented by Rai et al.

FIG. 1 illustrates a basic biasing structure used in prior art memorydevices. The memory devices include a plurality of sense amplifiers,such as sense amplifiers 22, 23, 24 and 25. In the illustrated system,sense amplifier 22 has a first input coupled to bit line BL0 and asecond input coupled to a reference voltage on line 26. Sense amplifier23 has a first input coupled to bit line BL1 and a second input coupledto a reference voltage on line 27. Sense amplifier 24 has a first inputcoupled to bit line BL2 and a second input coupled to a referencevoltage on line 28. Sense amplifier 25 has a first input coupled to bitline BL127 and a second input coupled to a reference voltage on line 29.As shown, a number of sense amplifiers, such as 8, 16, 32, 64, theillustrated 128, or more, for sensing a number of bits of data inparallel from a memory array are provided. Decoding circuitry, notshown, is used to select, from among thousands of columns of memorycells in a typical integrated circuit memory device, columns of memorycells for connection to the bit lines in response to addressing. In FIG.1, memory cell 30 is shown coupled to bit line BL0, memory cell 31 isshown coupled to bit line BL1, memory cell 32 is shown coupled to bitline BL2 and memory cell 33 is shown coupled to bit line BL127. Acapacitance C_(BL) is illustrated in the figure, which represents thetotal bit line capacitance for a path on the bit line BL0 to a selectedmemory cell on a selected column in the memory array. The voltage on thebit line V_(BL), is established by charging the bit line capacitanceC_(BL) with current through the load transistors 14-17.

Each of the bit lines BL0, BL1, BL2, . . . BL127 includes a biasingstructure used to bias the bit line in preparation for sensing. Thebiasing structure in the illustrated embodiment for the bit line BL0comprises a clamp transistor 10 and a load transistor 14. Clamptransistor 11 and load transistor 15 are coupled with bit line BL1.Clamp transistor 12 and load transistor 16 are coupled with bit lineBL2. Clamp transistor 13 and load transistor 17 are coupled with bitline BL127. The load transistors 14-17 in the illustrated embodimentcomprise respective n-channel MOS transistors having their gates anddrains coupled to a supply potential VDD, and sources coupled to asensing node (labeled V_(CELL) at the sense amplifier inputs) on therespective bit lines. The clamp transistors 10-13 comprise respectiven-channel MOS transistors having drains coupled to the sensing nodes(V_(CELL)), sources coupled to conductors that are coupled via decodingcircuitry to the selected memory cells in the array, and gates coupledto the output of respective feedback inverters 18-21. The inputs to thefeedback inverters 18-21 are coupled to the sources of the clamptransistors 10-13 and to the conductors that are coupled via decodingcircuitry to the selected memory cells in the array.

In the operation, the load transistors and clamp transistors maintainthe sensing node at a level designed to match the sense amplifieroperation, and to allow fast sensing. Thus, the sensing node ismaintained at a level usually just above the reference voltage (labeledV_(REF) at the sense amplifier inputs). With reference to bit line BL0,as the voltage V_(BL) on the bit line BL0 at the source of the clamptransistor 10 reaches a level at the trigger point of the feedbackinverter 18, the output of the feedback inverter 18 starts to go down,and begins to turn off the clamp transistor 10, which reduces currentflow and tends to allow the voltage on the source of the clamptransistor 10 to stop rising, or go down. An equilibrium condition isestablished on the bit line BL0 by this dynamic feedback, with a smallcurrent through the load transistor 14. The voltage at the sensing nodeV_(CELL) settles at the target level, and the bit line is ready forsensing. After the interval allowing the voltage at the sensing node tosettle at the target level, the memory cell is accessed for sensing byapplying a word line potential to the gate of the memory cell, forexample, on a selected word line. If the memory cell turns on inresponse to the voltage applied on its gate, then the voltage on thesensing node will be pulled down below the reference voltage V_(REF). Onthe other hand, if the memory cell does not turn on in response to thevoltage applied on its gate, then the voltage on the sensing node willnot be pulled down. The sense amplifier determines how the voltage onthe sensing node behaves and generates an output signal indicating thevalue of the data stored in the memory cell.

In an alternative embodiment known in the prior art, as shown in FIG. 2,the dynamic feedback inverters are replaced with a simple bias voltageV_(BIAS). Thus, the embodiment shown in FIG. 2 includes load transistor40 and clamp transistor 41, which are arranged like load transistor 14and clamp transistor 10 of FIG. 1. The bias voltage V_(BIAS) is producedby a reference voltage circuit, and applied to the gate of the clamptransistor 41. The source of the clamp transistor 41 is coupled to aselected memory cell through decoding circuitry not shown. The bit linecapacitance is represented by the capacitor C_(BL) as discussed abovewith respect to FIG. 1. The sensing node between the load transistor 40and the clamp transistor 41 is coupled to a sense amplifier 42. Thecircuit in FIG. 2 operates in a manner similar to that described abovewith respect to FIG. 1, without the dynamic feedback. As the voltageV_(BL) on the bit line reaches a level that is about a threshold voltagedrop across the clamp transistor 41 below the bias voltage V_(BIAS), theclamp transistor 41 begins to turn off and reduce current flow. Thedynamic balance is achieved with the voltage at the sensing nodeV_(CELL) settling on a target value. At this point, the precharge stepis completed, and the bit line is ready for sensing. Upon accessing amemory cell, the cell data influences the voltage at the node V_(CELL),causing it to move quickly toward a high cell threshold value V_(CELL)HVT or toward a low cell threshold value V_(CELL) _(—) _(LVT). Thereference voltage V_(REF) applied to the sense amplifier 42 is set at avalue about halfway between V_(CELL) _(—) _(HVT) and V_(CELL) _(—)_(LVT). The margin between the target value on V_(CELL) and V_(REF) atthe sense amplifier 42 is large enough to cover noise effects, but assmall as possible for quick sensing.

The time required for the voltage on the sensing node to settle at itstarget level has limited the speed of such sensing systems. Thus,precharging techniques have been provided such as shown in FIG. 3, whichapply a higher current during the process of raising the bit linevoltage V_(BL) to establish the target level at the sensing node. In theprior art embodiment of FIG. 3, a bit line is coupled to a selectedmemory cell 53 by decoding circuitry not shown. The clamp transistor 51on the bit line is connected to the sensing node V_(CELL). A load 50(such as the diode connected transistor shown in FIG. 1 and FIG. 2, butalso other types of loads could be provided), is connected between thesensing node V_(CELL) and a supply potential VDD. Sense amplifier 52 iscoupled to the sensing node V_(CELL) and a reference voltage V_(REF), asdescribed above. The gate of the clamp transistor 51 is connected tobias voltage V_(BIAS) like that described with reference to FIG. 2. Inan alternative system, a dynamic feedback inverter, connected as shownin FIG. 1, is used to bias the gate of the clamp transistor 51.Additional precharge current is provided through transistor 54 andtransistor 55. Transistor 54 is an n-channel MOS transistor having itssource coupled to the source of clamp transistor 51, and its gatecoupled to the gate of clamp transistor 51 so that it receives the samebias voltage V_(BIAS) (or the same output of the feedback inverter).Transistor 55 is a p-channel MOS transistor having its drain coupled tothe drain of transistor 54, its source coupled to a precharge supplyvoltage, which is typically, although not necessarily, the same supplyvoltage as the load supply voltage VDD. The gate of the transistor 55 iscontrolled by a logic signal PRE, which enables precharging when it isat a low level, by turning on transistor 55 into saturation withconsequently very little voltage drop across it. Transistor 54 is atransistor having a higher threshold voltage than the clamp transistor51. The higher threshold is achieved for example by making transistor 54with a narrower and longer channel region. Therefore, during a prechargeinterval precharge paths are provided both through the load 50 and thetransistor 55. Both transistors 54 and 51 will be on while the voltageon the bit line V_(BL) is low. As the voltage on the bit line V_(BL)approaches V_(BIAS) (less the threshold of transistor 54, including bodyeffects), transistor 54 will turn off first because of its higherthreshold voltage, and disable the precharge path through transistor 55.Dynamic balance will be achieved between the load 50 and the clamptransistor 51 as described above, settling the sensing node at thetarget level. Because the path through transistor 55 is enabled duringthe first part of the precharge operation, more current is applied tocharging up the bit line capacitance C_(BL), and the voltage on the bitline V_(BL) rises more quickly. Thus, the sensing system settles on thetarget voltage more quickly. With a shorter precharge interval, fastersensing can be achieved.

While these prior art techniques have been applied for memory devicessuccessfully, as memory access speeds increase, component sizesdecrease, and more complicated and more highly parallel sensingstructures are deployed, the requirement of complex biasing structureson every bit line is becoming a limiting factor on size and cost ofintegrated circuit memories. It is therefore desirable to providesensing systems that occupy less space on an integrated circuit, operatefaster and consume less power.

SUMMARY OF THE INVENTION

An integrated circuit memory device is described including an array ofmemory cells with an extra column of memory cells and a reference bitline coupled to the extra column, in which the reference bit line actsas a reference for a shared clamping control bias on the bit lines inthe array. The memory device in an embodiment described herein includesan array of memory cells with a plurality of columns and rows. Aplurality of bit lines is coupled to the columns in the array and aplurality of word lines is coupled to the rows in the array. The extracolumn of memory cells and the reference bit line are included on thedevice, as part of, or as a component adjacent to, the memory array.Clamp transistors are coupled to respective bit lines in the pluralityof bit lines, and adapted to prevent voltage on the respective bit linesfrom exceeding a target level. A detector, such as a comparator, isprovided that has an input coupled to the reference bit line and anoutput coupled to the clamp transistors on the plurality of bit lines.

In a described embodiment, the detector generates a bias voltage on itsoutput which turns on the clamp transistors at a first bias level whenthe reference bit line has a voltage below the target level, and at asecond bias level, which is lower than the first bias level, when thereference bit line has a voltage near the target level. In this manner,the clamp transistors receive higher voltage during the first part ofthe precharge cycle and conduct more current to increase the speed ofprecharging the bit line. As the voltage on the reference bit line nearsthe target level, the detector switches the bias voltage to a lowerlevel, at which the clamp transistor is maintained in a clampingcondition, and a dynamic balance is achieved to prepare the bit line forsensing.

The reference bit line in an embodiment of the technology describedherein is arranged to emulate the timing of the bit lines in the arrayby, for example, structurally matching the bit lines in the array, sothat voltage changes during precharging on the reference bit line matchthose on the bit lines in the array that are being accessed for sensing.In an example, the extra column of memory cells, to which the referencebit line is coupled, includes the same number of memory cells as acolumn of memory cells in the array.

An embodiment is described which includes a reference clamp transistoron the reference bit line, and a reference load transistor on thereference bit line. The reference clamp transistor and the referenceload transistor are arranged to match corresponding clamp transistorsand load transistors on the bit lines in the array, so that voltages onthe reference bit line during precharging through the reference clamptransistor change at essentially the same rate of change as voltages onthe bit lines in the array for the purpose of the timing of theprecharging and clamping.

A sense amplifier suitable for use with the plurality of bit lines ofthe memory array, according to the embodiments described above, includesa dummy cell or an array of dummy cells which are arranged to emulatethe voltage threshold behavior of memory cells in the array. The dummycell is used to generate a reference voltage for the sense amplifier,which tracks changes in the threshold voltages of memory cells in thearray that occurred due to temperature changes and the like.

In general, a method for sensing data in a memory device is described,where the memory device comprises an array of memory cells including aplurality of columns and rows, a plurality of bit lines coupled tocolumns in the array and a plurality of word lines coupled to rows inthe array. The method includes emulating the timing of bit lines in theplurality of bit lines with an extra column of memory cells and areference bit line coupled to the extra column of memory cells. Nodes onrespective bit lines in the plurality of bit lines are clamping near atarget level with clamp transistors that are responsive to a biasvoltage. The bias voltage is generated based on timing on the referencebit line. In one embodiment, the bias voltage has a first bias levelwhen the reference bit line has a voltage below the target level, and ata second bias level, which is lower than the first bias level, when thereference bit line has a voltage near the target level.

According to the technology described herein, an extra reference bitline is coupled with a comparator, or other type of detector, togenerate a clamping bias for bit lines in an array of memory cells. Asingle detector is utilized instead of a large number of dynamicfeedback inverters, as described with reference to FIG. 1, which reducescurrent consumption in the sensing system and reduces layout area. Also,these advantages are provided along with fast precharging in embodimentsof the disclosed technology, provided by the dual bias voltagesgenerated by the output of the comparator, where the first bias voltageis used for enabling higher current flow through the clamp transistorsto improve the speed of precharging, and a second bias voltage is usedfor clamping bias to establish a dynamic balance for sensing.

Other aspects and advantages of the present invention can be seen onreview of the drawings, the detailed description and the claims, whichfollow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a prior art sensing circuit based on dynamic inverterfeedback.

FIG. 2 illustrates a prior art sensing circuit based on static biasvoltage clamping.

FIG. 3 illustrates a prior art sensing circuit including a fastprecharge circuit along with voltage clamping.

FIG. 4 illustrates a sensing circuit including an extra bit line andshared clamping bias.

FIG. 5 illustrates a sensing circuit including an extra bit line, sharedclamping bias, and fast precharge switching.

FIG. 6 is a simplified block diagram of an integrated circuit memorydevice including the technology described herein.

DETAILED DESCRIPTION

A detailed description of embodiments of the present invention isprovided with reference to the FIGS. 4-6.

FIG. 4 illustrates sensing circuitry including a reference bit line DMBL(dummy bit line) and shared biasing circuitry for high speed and lowpower precharging. A memory array is represented by memory cells100-102, found on respective columns in an array of memory cells, andselected for connection to bit lines by decoding circuitry not shown.Capacitor symbols C_(BL) are illustrated, and associated with each ofthe bit lines. The capacitor symbols C_(BL) represent the total bit linecapacitance for access to a selected cell. In the illustratedembodiment, there are N bit lines, labeled BL0, BL1, . . . BLN in thearray of memory cells. Clamp transistors 103-105 and load transistors106-108 are included on respective bit lines BL0-BLN, and arrangedidentically in the illustrated embodiment. Clamp transistor 103 acts asa clamping circuit on bit line BL0. In this embodiment, the clamptransistor 103 is an n-channel MOS transistor with a source coupled to aconductor which is in turn coupled via decoding circuitry to theselected memory cell, with a drain coupled to a sensing node V_(CELL),and a gate coupled to a biasing node V_(BIAS). Load transistor 106 actsas a load on bit line BL0. The load transistor is an n-channel MOStransistor with its drain and gate coupled to the supply potential VDD,and its source coupled to the sensing node V_(CELL). Clamp transistor104 and load transistor 107 on bit line BL1 are arranged in the sameway. Likewise, clamp transistor 105 and load transistor 108 on bit lineBLN are arranged in the same way. As illustrated, the clamp transistors103-105 have their gates coupled to a common node at the output of acomparator 120, which applies the bias voltage V_(BIAS).

The sensing node V_(CELL) on bit line BL0 is coupled to sense amplifier109. Likewise, the sensing node V_(CELL) on bit line BL1 is coupled tosense amplifier 110. The sensing node V_(CELL) on bit line BLN iscoupled to sense amplifier 111. Each of the sense amplifiers 109-111includes a second input coupled to a reference voltage V_(REF). Thesense amplifiers 109-111 provide output data D_(OUT), which indicatesthe data stored in the respective selected memory cells 100-102.

The reference bit line DMBL is arranged to emulate the timing behaviorof the bit lines BL0-BLN in the array. In the embodiment shown, thereference bit line DMBL is coupled to extra column of memory cells bydecoding circuitry, not shown, similar to that on the bit lines BL0-BLNin the array. A selected memory cell 121 in the extra column isillustrated in the figure, representing that the structure of the extracolumn of memory cells coupled to the reference bit line DMBL matchesthe structure of the columns of memory cells in the array. Likewise, thecapacitor symbol CDMBL is shown representing the capacitance of the pathto a selected memory cell 121 on the reference bit line DMBL. A clamptransistor 122 and a load transistor 123 are included on the referencebit line DMBL. The clamp transistor 122 is an n-channel MOS transistorwith a source coupled to a conductor in the reference bit line, which isin turn coupled via decoding circuitry, or other structures that accountfor the load on the bit lines in the array contributed by the decodingcircuitry in the array, to the selected memory cell 121. The drain ofclamp transistor 122 is coupled to a reference sensing node VDM, and agate is coupled to the biasing node V_(BIAS). Load transistor 106 onreference bit line DMBL is an n-channel MOS transistor with its drainand gate coupled to the supply potential VDD, and its source coupled tothe reference sensing node V_(DM). The reference sensing node V_(DM) inone embodiment is coupled to a dummy sense amplifier, not shown, orother structure having a load the like that of the load of actual senseamplifiers on the bit lines in the array, so that the behavior of thereference bit line emulates that of the actual bit lines for thepurposes of providing the clamping and precharge control as describedherein.

Comparator 120 has a first input (negative polarity) coupled to areference voltage V_(DET) and a second input (positive polarity) coupledto the source of the clamp transistor 122 on the reference bit line. Afirst supply potential, such as the potential VDD, and a second supplypotential VB, which is lower than the first supply potential and closeto a target precharge voltage on the bit lines, are applied to thecomparator 120. For example, in one embodiment VDD is about three volts,and VB is about 2.3 volts. The output of the comparator 120 is the biasvoltage V_(BIAS), and has a level close to the supply potential VDD whenthe reference bit line voltage is below the reference voltage V_(DET),and has a level near the second supply potential VB, when the voltage onthe reference bit line is above the reference voltage V_(DET). In theillustrated embodiment, the voltage V_(DET) is set at a level about VBminus the threshold of the clamp transistor 122. In this manner, as thevoltage on the reference bit line approaches VB minus the threshold ofthe clamp transistor 122, the reference bit line is balanced and thevoltage on the sensing node V_(DM) is stabilized. Likewise, the voltageV_(BIAS) is applied to the gates of the clamp transistors 103-105 on allof the bit lines BL0-BLN in the array. Shortly after the comparator 120switches the voltage V_(BIAS) from the supply potential VDD to thesupply potential VB, the precharge cycle in the array is completed, andthe bit lines BL0-BLN in the array are ready for sensing. Upon accessinga memory cell, the cell data influences the voltage at the nodeV_(CELL), causing it to move quickly toward a high cell threshold valueV_(CELL) _(—) _(HVT) or toward a low cell threshold value V_(CELL) _(—)_(LVT). The reference voltage V_(REF) applied to the sense amplifiers109, 110, 111 is set at a value about halfway between V_(CELL) _(—)_(LVT) and V_(CELL) _(—) _(LVT). The margin between the target value onV_(CELL) and V_(REF) at the sense amplifiers 109, 110, 111 is largeenough to cover noise effects, but as small as possible for quicksensing.

FIG. 5 illustrates another embodiment of sensing circuitry including areference bit line DMBL (dummy bit line) and shared biasing circuitryfor high speed and low power precharging. The sensing circuitry of FIG.5 is suited for higher-speed operation than that of FIG. 4, and forlarge numbers of bit lines. A memory array is represented by memorycells 200-202 found on respective columns in an array of memory cells.Selected memory cells are selected for connection to bit lines BL0-BL127by decoding circuitry not shown. Capacitor symbols C_(BL0)-C_(BL127) areillustrated, and associated with each of the bit lines. The capacitorsymbols CBLX represent the total bit line capacitance for access to aselected cell. In the illustrated embodiment, there are N=128 bit lines,labeled BL0, BL1, . . . BL127 in the array of memory cells. Clamptransistors 203-205 and loads 206-208 are included on respective bitlines BL0-BL127, and arranged identically in the illustrated embodiment.Thus, clamp transistor 203 acts as a clamping circuit on bit line BL0.The claims transistor 203 is an n-channel MOS transistor with a sourcecoupled to a conductor which is in turn coupled via decoding circuitryto the selected memory cell, with a drain coupled to a sensing node SA,and a gate coupled to a biasing node V_(BIAS). The sensing nodes SA arecoupled to sense amplifiers according to the particular sensingtechnology applied, including for example in the manner described abovewith reference to the other figures. Load 206 on bit line BL0 can beann-channel MOS transistor as described above, or other load circuits.Clamp transistor 204 and load 207 on bit line BL1 are arranged in thesame way. Likewise, clamp transistor 205 and load 208 on bit line BL127are arranged in the same way. As illustrated, the clamp transistors203-205 have their gates coupled to a common node at the output of acomparator 220, which applies the bias voltage V_(BIAS).

Precharge transistors 209, 210, 211 are coupled to respective bit linesBL0, BL1, BL127 in the embodiment of FIG. 5. The precharge transistors209-211 act as precharge circuits on the respective bit lines, and inthe illustrated embodiment comprise p-channel MOS transistors havingtheir sources coupled to a precharge supply, such as the supplypotential VDD or other suitable precharge supply potential, their drainscoupled to the sensing nodes SA, or other suitable nodes, on therespective bit lines BL0-BL127, and their gates coupled to a prechargecontrol signal PRE which is generated at the output of the detector 226.Thus, during assertion of the control signal PRE, which is active low inthis embodiment, precharge current is applied to the bit line via theprecharge transistors 209-211. Also, the precharge transistors 209-211operate based on shared control circuitry that is responsive to thereference bit line DMBL.

The reference bit line DMBL is arranged to emulate the timing behaviorof the bit lines BL0-BL127 in the array. In the embodiment shown, thereference bit line DMBL is coupled to an extra column of memory cells bydecoding circuitry, not shown, similar to that on the bit linesBL0-BL127 in the array. A selected memory cell 221 in the extra columnis illustrated in the figure, representing that the structure of theextra column of memory cells coupled to the reference bit line DMBLmatch the structure of the columns of memory cells in the array.Likewise, the capacitor symbol C_(DMBL) is shown representing thecapacitance of the path to a selected memory cell 221 on the referencebit line DMBL. A clamp transistor N1 and a load 223 are included on thereference bit line DMBL. The clamp transistor N1 is an n-channel MOStransistor, substantially matching the clamp transistors 203-205 in thearray, with a source coupled to a conductor in the reference bit line,which is in turn coupled via decoding circuitry to the selected memorycell 221, with a drain coupled to a load 223 and a gate coupled to thebiasing node V_(BIAS). Load 223 on reference bit line DMBL matches theloads 206-208 in the array. The node at the drain of clamp transistor N1in one embodiment is coupled to a dummy sense amplifier, not shown, orother structure having a load the like that of the load of an actualsense amplifier on the bit lines in the array, matching as closely asnecessary so that the behavior of the reference bit line emulates thatof the actual bit lines for the purposes of providing the clamping andprecharge control as described herein.

A precharge transistor 225 is included which matches the prechargetransistors 209-211 on the bit lines BL0-BL127 in the array. The controlsignal PRE for the precharge transistors 225, and 209-211 is generatedby the detector 226. The detector 226 is coupled to a second clamptransistor N2, which is in turn coupled via a load 224 to the supplypotential VDD. Thus the second clamp transistor N2 has a source coupledto the source of the first clamp transistor N1, a gate coupled to thenode V_(BIAS), and a drain coupled to the load 224. The drain of thesecond clamp transistor N2 is coupled to the input of the detector 226,which switches the control signal PRE from the low state to the highstate, turning off the precharge transistors 225 and 209-211, when thevoltage on the drain of second clamp transistor N2 reaches the detectorthreshold.

Comparator 220 has a first input (negative polarity) coupled to areference voltage V_(DET) and a second input (positive polarity) coupledto the source of the clamp transistors N1 and N2 on the reference bitline. A first supply potential, such as the potential VDD, and a secondsupply potential VB, which is lower than the first supply potential, andclose to a target precharge voltage on the bit lines, are applied to thecomparator 220. For example, in one embodiment VDD is about three volts,and VB is about 2.3 volts. The output of the comparator 220 is the biasvoltage V_(BIAS), and has a level close to the supply potential VDD whenthe reference bit line voltage is below the reference voltage V_(DET),and has a level close to the second supply potential VB, when thevoltage on the reference bit line is above the reference voltageV_(DET). In the illustrated embodiment, the voltage V_(DET) is set at alevel which is about VB minus the threshold of the clamp transistor N1.In this manner, as the voltage on the reference bit line approaches VBminus the threshold of the clamp transistor N1, the reference bit lineis balanced and the voltage on the sensing node V_(DM) is stabilized.Likewise, the voltage V_(BIAS) is applied to the gates of the clamptransistors 203-205 of all of the bit lines BL0-BL127 in the array.Shortly after the comparator 220 switches the voltage V_(BIAS) from thesupply potential VDD to the supply potential VB, the precharge cycle inthe array is completed, and the bit lines BL0-BLN in the array are readyfor sensing. Upon accessing a memory cell, the cell data influences thevoltage at the node SA, causing it to move quickly toward a high cellthreshold value V_(CELL) _(—) _(HVT) or toward a low cell thresholdvalue V_(CELL) _(—) _(LVT). Sense amplifiers (not shown) are implementedfor example, as described above with reference to FIG. 4.

The first clamp transistor N1 and the second clamp transistor N2 aresimilar devices, except that the second clamp transistor N2 is arrangedso that it has a slightly higher threshold voltage than the first clamptransistor N1 in the illustrated embodiment. For example, the secondclamp transistor N2 has a longer and narrower channel region than thefirst clamp transistor N1. In this manner, the second clamp transistorN2 will turn off slightly before the first clamp transistor N1, causingthe input to the detector 226 to rise quickly, trigger the detector 226,and thereby turn off the precharge transistors. Thus, the prechargetransistors 209-211 are turned off before the dynamic balancing of theclamp transistors 203-205 is finished, to avoid over-charging. A fasterprecharge operation is achieved because of greater current provided bythe precharge transistors parallel with the loads on the bit lines inthe array, during the beginning of the precharge cycle.

It is noted that the clamp circuits and precharge circuits in theillustrated embodiments are made with single transistors. Although otherembodiments may include additional circuit elements, single transistorembodiments of the clamp circuits and precharge circuits are efficientin power consumption, size and speed.

FIG. 6 is a simplified block diagram of an integrated circuit includingprecharge and clamping circuitry controlled by a reference bit line. Theintegrated circuit includes a memory array 600 implemented using memorycells, such as floating gate or charge trapping non-volatile memorycells, read only memory cells, or other types of memory cells. Apage/row decoder 601 is coupled to a plurality of word lines arrangedalong rows in the memory array 600. A column decoder 603 is coupled to aplurality of bit line conductors 604 arranged along columns of memorycells in the memory array 600. Clamping/bias structures 623 are coupledto the columns of memory cells in the memory array via the columndecoder 603 and bit line conductors 604 in the manner described above,including for example in the manner described with respect to FIG. 4 orFIG. 5. Also, precharge structures 633 are coupled to the columns ofmemory cells in the memory array via the column decoder 603 and bit lineconductors 604 in the manner described above, including for example inthe manner described with respect to FIG. 5. An extra column 602 ofmemory cells is included in the memory array 600. The extra column canbe formed adjacent to the memory array or elsewhere on the integratedcircuit in alternative embodiments. A reference bit line conductor 614is coupled to circuitry 613 including shared control circuitry andmatching components for the column decoder circuitry 603 in the array,circuitry 624 including shared control circuitry and matchingcomponents, the clamp/bias structures 623 in the array and circuitry 634including shared control circuitry, and matching components for theprecharge structures 633 in the array.

Addresses are supplied on bus 605 to column decoder 603 (and circuitry613) and page/row decoder 601. Sense amplifiers and data-in structuresin block 606 are coupled to the selected memory cells via the columndecoder 603, clamp/bias structures 623, and precharge structures 633. Aplurality of reference dummy cells 640 is included on the integratedcircuit, and used for generating reference voltage which is used by thesense amplifiers in the block 606, so that the reference voltage used bythe sense amplifiers in the block 606 track changes in threshold of theactual memory cells in the memory array 600. Data is supplied via thedata-in line 611 from input/output ports on the integrated circuit tothe data-in structures in block 606. Data is supplied via the data-outline 612 from the sense amplifiers in block 606 to input/output ports onthe integrated circuit.

Resources for controlling the reading, programming and erasing of memorycells in the array 600 are included on the chip. These resources includeread/erase/program supply voltage sources represented by block 608, andthe state machine 609, which are coupled to the array 600, the decoders601, 603 and other circuitry on the integrated circuit, whichparticipates in operation of the device.

The supply voltage sources (block 608) are implemented in variousembodiments using charge pumps, voltage regulators, voltage dividers andthe like as known in the art, for supplying various voltage levels,including negative voltages, used in the read, erase and programoperations.

The state machine 609 supports read, erase and program operations. Thestate machine 609 can be implemented using special-purpose logiccircuitry as known in the art. In alternative embodiments, thecontroller comprises a general-purpose processor, which may beimplemented on the same integrated circuit, which executes a computerprogram to control the operations of the device. In yet otherembodiments, a combination of special-purpose logic circuitry and ageneral-purpose processor may be utilized for implementation of thestate machine.

While the present invention is disclosed by reference to the preferredembodiments and examples detailed above, it is to be understood thatthese examples are intended in an illustrative rather than in a limitingsense. It is contemplated that modifications and combinations willreadily occur to those skilled in the art, which modifications andcombinations will be within the spirit of the invention and the scope ofthe following claims.

1. A memory device, comprising: an array of memory cells, including aplurality of columns and rows; a plurality of bit lines coupled tocolumns in the array; a plurality of word lines coupled to rows in thearray; an extra column of memory cells and a reference bit line coupledto the extra column of memory cells; clamp circuits, coupled torespective bit lines in the plurality of bit lines, and adapted toprevent voltage on the respective bit lines from exceeding a targetlevel; and a detector having an input coupled to the reference bit lineand an output coupled to the clamp circuits, the detector generating abias voltage on its output which turns on the clamp circuits with afirst bias level when the reference bit line has a voltage below thetarget level, and with a second bias level when the reference bit linehas a voltage near the target level.
 2. The device of claim 1, whereinthe clamp circuits include a clamp transistor having a source coupled tothe reference bit line, a gate connected to receive the bias voltage,and a drain coupled through a load to a supply potential.
 3. The deviceof claim 1, wherein the second bias voltage is lower than the first biasvoltage.
 4. The device of claim 1, wherein the second bias voltage islower than the first bias voltage, and the second bias voltage has abias level near the target level plus a voltage drop across the clampcircuits, which maintains the clamp circuits in a clamping condition. 5.The device of claim 1, wherein the reference bit line is adapted toemulate timing behavior in the bit lines in the plurality of bit lines.6. The device of claim 1, wherein the array has a number N rows ofmemory cells and the extra column includes the number N memory cells. 7.The device of claim 1, including a set of sense amplifiers, senseamplifiers in the set including respective first inputs coupled toselected bit lines in the plurality of bit lines, and respective secondinputs coupled to reference circuits.
 8. The device of claim 1,including a set of sense amplifiers, sense amplifiers in the setincluding respective first inputs coupled to selected bit lines in theplurality of bit lines, and respective second inputs coupled toreference memory cells, the reference memory cells adapted to emulatebehavior of threshold voltages in memory cells in the array.
 9. Thedevice of claim 1, wherein said detector comprises a comparator, havingone input coupled to a node on the reference bit line, and another inputcoupled to a reference voltage.
 10. The device of claim 1, including areference clamp circuit including a reference clamp transistor having agate, a source coupled to a clamping node on the reference bit line, anda drain coupled to a load circuit on the reference bit line, saiddetector comprises a comparator, having one input coupled to the sourceof the reference clamp transistor, another input coupled to a referencevoltage, and an output coupled to the gate of the reference clamptransistor and to said clamp circuits.
 11. The device of claim 1,wherein said array of memory cells comprises non-volatile memory cells.12. A method for sensing data in a memory device, comprising an array ofmemory cells, including a plurality of columns and rows; a plurality ofbit lines coupled to columns in the array; and a plurality of word linescoupled to rows in the array; the method comprising: emulating thetiming of bit lines in the plurality of bit lines with an extra columnof memory cells and a reference bit line coupled to the extra column ofmemory cells; clamping nodes on respective bit lines in the plurality ofbit lines near a target level with clamp circuits responsive to a biasvoltage; and generating the bias voltage based on timing on thereference bit line, the bias voltage having a first bias level when thereference bit line has a voltage below the target level, and having asecond bias level when the reference bit line has a voltage near thetarget level.
 13. The method of claim 12, wherein the second biasvoltage is lower than the first bias voltage.
 14. The method of claim12, wherein the second bias voltage is lower than the first biasvoltage, and the second bias voltage has a bias level near the targetlevel plus a voltage drop across a clamp transistor in a clamp circuitwhich maintains the clamp circuits in a clamping condition.
 15. Themethod of claim 12, wherein the array has a number N rows of memorycells and the extra column includes the number N memory cells.
 16. Themethod of claim 12, including sensing voltage or current on selected bitlines in the plurality of bit lines after bias voltage has the secondbias level.
 17. The method of claim 12, including sensing voltage orcurrent on selected bit lines in the plurality of bit lines after biasvoltage has the second bias level, by comparison with voltage or currentthrough a reference memory cell.
 18. The method of claim 12, includingcomparing the voltage on the reference bit line with a referencevoltage, and generating the bias voltage in response to the comparison.19. The method of claim 12, wherein said array of memory cells comprisesnon-volatile memory cells.
 20. The method of claim 12, wherein saidclamp circuits include a clamp transistor having a source coupled to thereference bit line, a gate connected to receive the bias voltage, and adrain coupled through a load to a supply potential.
 21. A memory device,comprising: an array of memory cells, including a plurality of columnsand rows; a plurality of bit lines coupled to columns in the array, bitlines in the plurality respectively including a load between a supplyvoltage and a sensing node, and a clamp transistor having a gate, adrain coupled to the sensing node and a source coupled to a conductorcoupled to the corresponding columns in the array; a plurality of wordlines coupled to rows in the array; an extra column of memory cells anda reference bit line coupled to the extra column of memory cells, thereference bit line including a load between a supply voltage and areference node, and a reference clamp transistor having a gate, a draincoupled to the reference node and a source coupled to a conductorcoupled to the extra column of memory cells; and a comparator having aninput coupled to the source of the reference clamp transistor, a secondinput coupled to a reference voltage, and an output coupled to the gatesof the clamp transistors on the plurality of bit lines, and to the gateof the reference clamp transistor, the comparator generating a biasvoltage on its output which turns on the clamp transistors with a firstbias level when the source of the reference clamp transistor has avoltage below the reference voltage, and with a second bias level whenthe source of the reference clamp transistor has a voltage near thereference voltage.
 22. The device of claim 21, wherein the second biaslevel is near to a sum of a threshold voltage for the clamp transistorson the plurality of bit lines and the reference voltage, and the firstbias voltage is near the supply potential.